Multiple-input multiple-output (MIMO) technologies are now commonly used in wireless communications systems to improve throughput, performance, or both. The main limitation of MIMO systems in practice is related to their implementation complexity, which increases linearly with the number of antennas. A class of MIMO, which is known as Spatial Modulation (SM), was introduced for wireless systems in which the number of RF chains in the transmitter is smaller than the number of antennas. Since the number of RF chains is restricted, all antennae cannot be active simultaneously. The basic principle of SM comprises selecting the indexes of the active antennae using information bits. Additional information bits are transmitted through the symbols transmitted from the active antennae.
As not all transmit antennae are active at the same time, SM suffers from limited throughput compared to spatial multiplexing (SMX), which transmits symbols in parallel from all of the antennae.
SM techniques provide a limited throughput compared to SMX when the number of transmit antennae are the same. In fact, from the perspective of throughput, SMX provides the upper limit for all MIMO techniques. A disadvantage of SMX is limited performance and high decoder complexity.
There is, therefore, a need to provide a more efficient method of spatial multiplexing.